The invention relates to electronic sewing machines in which the lateral swinging movement of the needle and the fabric feeding movement are electronically controlled, and more particularly to a device for such a type of sewing machine for automatically setting the tensions of the upper and lower threads to adequate values in accordance with the selected stitch patterns of different types, besides the manual adjustment of the upper and lower threads.
According to the prior art, the tensions of the upper and lower threads of a sewing machine are generally adjusted firstly by manually appropriately determining the tension of the lower thread in dependence upon the thickness of a fabric to be sewn, and then by setting the tension of the upper thread in such a manner that the upper thread tensioning device is manually operated to provide a provisional upper thread tension, with which the fabric is actually sewn to see if the upper and lower threads are interlocked substantially in the intermediate point of the thickness of the fabric. Some of the conventional sewing machine are provided with a device for automatically adjusting only the upper thread in accordance with the selected stitch patterns of different types. It is, however, to be noted that the optimum value of the upper thread tension should be drawn out of the consideration with respect to the lower thread tension.
This invention has been provided to eliminate the defects and disadvantages of the prior art. It is a primary object of the invention to automatically and appropriately adjust the tensions of the upper and lower threads of a sewing machine in accordance to the selected patterns of different types, and in accordance to the thicknesses of the fabrics to be sewn. More particularly, it is another object of the invention to automatically adjust the tensions of the upper and lower threads of the sewing machine in accordance to a selected pattern, for example, a pattern of pin-tack stitches which is formed by the twin needles accompanied by shrinks of the fabric between the two stitch lines, and in accordance to another pattern, for example, a button hole of pearl stitches as shown in FIG. 6 which is formed in such a manner that the tension of the lower thread is weaker than that of the upper thread, so that the interlocking points of the upper and lower threads may be exposed on the upper face of the fabric and the loops of the lower thread may hem up the cut-out buttonhole edges to consolidate the same.
The other features and advantages of the invention will be apparent from the following description of the preferred embodiment in reference to the attached drawings.